Recently, navigation systems for displaying a current vehicle position on a map have been widely adopted in vehicles. Such navigation systems require map data. The map data for navigation systems are usually recorded in a data storage unit, for example, a CD-ROM or DVD included in the navigation system, such that regional map data are retrieved from the data storage unit.
Another scheme for obtaining regional map data of a current vehicle position is to receive the regional map data from a separate map-server for transmitting such map data through a network, which is enabled by communication technology.
In the latter scheme, the entire map stored in the map-server is divided into a multitude of cells, and the navigation system of a vehicle partially downloads, from the map-server, data from cells (referred to as “cell data” hereinafter) that are adjacent to a cell in which the vehicle is currently positioned.
In general, in conventional systems, with map data divided into an orthogonal grid, when the vehicle position lies in any given cell, there are eight adjacent cells about which map data must be transmitted. Conventionally, the amount of data may be reduced by transmitting map data for cells lying along an expected trajectory of the vehicle. This may reduce the number of cells about which data must be transmitted to about five. However, transmission of map data through a wireless network still requires a substantial amount of communication time and load. Therefore, it is preferable that the amount of data required to be transmitted is reduced to a minimum.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.